Optical HR Sensor Bug

Interesting issue.

I have a Smart Trainer that can act as a HR sensor bridge. I don't have HR setup to use that. My Forerunner 955 only uses the built in optical HR sensor, which works extremely well for me.

If I pair the Smart Trainer, then only during an activity, my watch records and displays ZERO for my Heart Rate. When I stop the activity, it goes back to a valid optical HR reading.

I understand what is happening. But it seems like the watch should ignore an obvious bad HR connection (zeros) and use the optical HR sensor.

I think this is also an issue if a HR strap is paired and connected but not worn. Polar HR straps can continue to pair when sitting on a shelf and report zero bpm, for example.

0 eliobell 2 days ago

I found the solution for perfect wrist readings, and with this solution,
my 965 is extremely accurate!!!
You have to wear the watch upside down, with the sensor resting on your veins, and quite tightly on the inside of your wrist!
I can guarantee it doesn't miss a beat, even at rest or at night!
It may seem uncomfortable at first, but I quickly got used to it, and it records perfectly!

0 flowstate 2 days ago

I looked at the ANT+ HRM device profile docs, and instant heart rate is transmitted as a single byte (i.e. an unsigned integer in the range 0-255). 0 is the invalid value, and 1-255 are valid values "to be displayed by the display device without further interpretation".

See pg 15 of https://err.no/tmp/ANT_Device_Profile_Heart_Rate_Monitor.pdf

So I tend to agree with this post, although I think the title isn't great (the post isn't about a bug in the optical HR sensor at all).

If an ANT+ HRM is transmitting 0, then that means it isn't reading a valid heart rate. At the very least, the watch should record *nothing* / NULL / invalid for 0s received from an HRM, as opposed to literally 0. At best, it should switch the built-in optical HR if possible.

I will also point out that if dynamic source switching is available and enabled (which it is by default on currently supported watches, including FR955), then the watch *will* switch to optical HR when a compatible Garmin strap thinks it has a bad connection (between the user and strap).

Unfortunately, it doesn't seem like it will do so in the case that you described (these aren't compatible Garmin HR straps).

Of course, if the wireless connection (ANT+/bluetooth) between any external HRM and the watch is interrupted (e.g. signal dropout, dead battery), then the watch will switch to optical HR (but that doesn't help in this case).

This just seems like one of those edge cases that Garmin doesn't care about.